Add Configuring tasks link to the header nav bar

[leobalter]

Note that I slightely changed the icons:

It partially fixes the #133 commit.

[leobalter]

This is better done with the gruntjs/grunt-docs#94 to generate the complementing links for the API page after removing it from the header.

[JKVeganAbroad]

One thing makes me hesitate on merging this pull request:

• You're removing the API link from the header

Three things make me merge it anyway:

• API can be accessed through the "Documentation" link instead
• API is essentially a subset of "Documentation"

• 143 Suggests the API link is the least clicked link in the header