default-function: moved from 'features' to 'functions'. reworked `default` ref. to meet generic less-docs style, tried to clean it up by removing too dumb examples and too scary info.
'mixin-guards' section: reformatted examples, added a note about `default` function - needs editing.
Merge pull request #61 from seven-phases-max/next

Mixin guards `default` function
fixed a typo in nav-functions.hbs
Functions: added missing 'Type' functions (i.e. isnumber, iscolor etc.)
Type functions: fixed `isunit` examples.
Merge pull request #62 from seven-phases-max/type-funtions

Added missing 'Type' funtions