Added pagination parameters for all API calls where it is supported

Fixes issue #15

Fix documentation about ListProjectIssues method

Embed ListOptions in for issues && merge_requests

This enables setting page/per_page options in ListIssuesOptions &&
ListProjectIssueOptions && ListMergeRequestOptions.

Improve error message

Don't add the host into the url opaque param

Adding the host into the URL.Opaque parameter creates an absolute
uri request like:

    GET https://gitlab.com/api/v3/... HTTP/1.1

instead of:

    GET /api/v3/... HTTP/1.1

Apache seems to have a problem with absolute uri requests when used
as a reversed proxy, double encoding %2F to %252F before passing the
request to GitLab, which breaks GitLab's api that accepts NAMESPACE/PROJECT.

The proxy config used is:

    ProxyPass / http://127.0.0.1:8080/ nocanon

'nocanon' parameter is the one that prevents double encoding, but it
seems to work only for relative uri requests.

An alternative is to use the mod_rewrite module directly instead of
the ProxyPass directive as:

    RewriteEngine On
    RewriteRule ^(.*) http://127.0.0.1:8080$1 [NE,P]

with the 'NE' flag (no escape) which works fine with absolute uri
requests. There are performance issues with this method as explained
here http://httpd.apache.org/docs/current/rewrite/flags.html#flag_p

Add projects to group

Some fixes

Remove some unused fields of the Commit struct

Return a pointer instead

This way you can keep using the pointer methods on *url.URL as expected.

Add func to retrieve the baseURL

Adding Travis file...

First release v0.1.0