Add a contributing guide

Adds a contributing guide to the repository to aide would be contributors.
    Add gitignore file, remove force binary and metadata folder

    This adds a `.gitignore` file based off of [this example Go gitignore][1].

    This also removes the `force` artifact, and the metadata folder is from `force export`,
    adding both to `.gitignore`.

    [1]: https://github.com/github/gitignore/blob/master/Go.gitignore
Provide a better error message with expired OAuth token

Without refresh tokens, calls to `force export` (and probably others) can error
out with `ERROR: Bad_Oauth_Token`.

This pull request adds an path for handling the 403 returned and gives a better
error message
Run go fmt
Add step about running go fmt
Refactor runFieldCreate
Add support and fixed bug in sobject create.

There was a bug with field creation due to change in default required
field values. New command is now
force sobject create ObjectName FieldName:FieldType
[fieldattribute:attributevalue…]
Merge pull request #54 from devangel/master

Support for optional field attributes cause bug in sobject create -- fixed.