android: added development readme
preferences: make gap time configurable

Instead of using a constant 45min gap, put it in the preferences.

This still needs some work: the change in gap time isn't reflected
immediately, only after the next ping fires.
Merge pull request #26 from jeffkaufman/build-instructions

android: added development readme
Merge pull request #28 from jeffkaufman/add-gap-pref

preferences: make gap time configurable
Merge pull request #29 from temporaer/master

PyTagTime: fixed time averaging and day-of-the-week handling; nicer plots and more control of what to show
pytagtime: plot title shows date range
pytagtime: correct nan handling for top-n tags
pytagtime: improve pie-chart absolute values if some weekdays are omitted