Allow running Tilestream without HOME in the environment (fixes #155)

[wrigby]

This falls back to serving tiles from ./tiles in case the HOME environment variable is undefined, which makes it easier to run Tilestream under a process supervisor or from an init script (see #155).

I'm not 100% sure this is the desired behavior in a situation like this, but I figured I would get a PR up as soon as possible.

Not-so-exhaustive testing:

$ node --harmony index.js start --help
Usage: node index.js <command> [options...]
Commands: start application
  start

Options:
    --host               Hostnames allowed for requests. Wildcards are allowed. (Default: ["127.0.0.1","localhost","<redacted>"])
    --uiPort=[port]      UI server port. (Default: 8888)
    --tilePort=[port]    Tile server port. (Default: 8888)
    --tileHost=[host]    Tile hostname (Default: undefined)
    --subdomains=[list]  Comma separated list of subdomains to use for tiles. (Default: undefined)
    --tiles=[path]       Path to tiles directory. (Default: "/home/users/wrigby/Documents/MapBox/tiles")
    --accesslog          Print every request to stdout. (Default: false)
    --config=[path]      Path to JSON configuration file.
$ unset HOME
$ node --harmony index.js start --help
Usage: node index.js <command> [options...]
Commands: start application
  start

Options:
    --host               Hostnames allowed for requests. Wildcards are allowed. (Default: ["127.0.0.1","localhost","<redacted>"])
    --uiPort=[port]      UI server port. (Default: 8888)
    --tilePort=[port]    Tile server port. (Default: 8888)
    --tileHost=[host]    Tile hostname (Default: undefined)
    --subdomains=[list]  Comma separated list of subdomains to use for tiles. (Default: undefined)
    --tiles=[path]       Path to tiles directory. (Default: "/usr/lib/node_modules/tiles")
    --accesslog          Print every request to stdout. (Default: false)
    --config=[path]      Path to JSON configuration file.

[tmcw]

Closing; tilestream is deprecated in favor of any of many mvt servers - feel free to fork if you need an essential feature or switch to an alternative.