Send EmailsOnPush email when branch or tag is created or deleted.

Addresses #1951, #1957 and #1925.

![Screen_Shot_2015-03-17_at_13.58.15](https://dev.gitlab.org/gitlab/gitlabhq/uploads/16ff25adb4b4a7e1923612e0652442b4/Screen_Shot_2015-03-17_at_13.58.15.png)

![Screen_Shot_2015-03-17_at_13.58.22](https://dev.gitlab.org/gitlab/gitlabhq/uploads/e346c1d84aba3a093b722d0a4167e289/Screen_Shot_2015-03-17_at_13.58.22.png)

![Screen_Shot_2015-03-17_at_13.58.28](https://dev.gitlab.org/gitlab/gitlabhq/uploads/720437ecc13f317c6d20eff82ac60bd7/Screen_Shot_2015-03-17_at_13.58.28.png)

![Screen_Shot_2015-03-17_at_13.58.34](https://dev.gitlab.org/gitlab/gitlabhq/uploads/2b302bb6cdbe27c96a8dff1375236602/Screen_Shot_2015-03-17_at_13.58.34.png)

See merge request !1709