0.13.1 - bugfixes

Fixes and improvements including

• It's now possible to .yotta_ignore CMake files (useful when adopting existing libraries to yotta)
• Warn when a target has not been explicitly set
• Warn when using yotta link to link a module which is not a dependency
• Fix backtrace under python 3 when setting an invalid target
• Fix case where yotta test could hang indefinitely (thanks @bridadan!)
• Always use the full path to import sub packages (yotta no longer modifies sys.path), which should improve compatibility across different python installations.
• Update and simplify Ubuntu installation instructions
• Various other documentation fixes and improvements