Migrated compiled binaries to Github releases. Fixes #479

Signed-off-by: M. Mucahid Benlioglu <m.mucahidbenlioglu@gmail.com>

auth/file_source: don't try to read non-existent files

Commit b33b3e96 introduced a bug where we try to read a token file even if
it doesn't exist, causing unauthenticated run (e.g.  `gdrive about`) to fail
with the error

    Failed getting oauth client: Failed to read token: unexpected end of JSON input

Closes #257.

clarify that only the root .gdriveignore is used

New feature, support authentication via service account.

Added --description option

Fix #159

Document how to list files in a subdirectory.

Add the skip parameter to download and download query commands

add the parameter to specify the Google Apps domain when the sharing type is domain

Add --mime option to gdrive import

Fixed typo

`directoy` -> `directory`

add Homebrew installation instructions

as now https://github.com/Homebrew/homebrew-core/pull/754
has been accepted we do not to tap

add Go dependencies by godep

see https://github.com/tools/godep

Clarifies the phrase 'first time gdrive is launched'.

Newbie users, like myself, compiled several versions of gdrive because we were not getting the auth. url when running `gdrive` from terminal as pointed out by the phrase "The first time gdrive is launched ... you will be prompted for a verification code".

Make `go get` work