Refactor internal paths handling

Since `@root_path`, `@data_path` and `@repos_path` are evaluated at once, user have to reset not only `@root_path` but also `@data_path`.
This commit stops to use these instance variables and add accessor methods to `Tachikoma` module.
As a result of this commit, user can customize all internal paths by to reset only `Tachikoma.root_path`.

Usage:

    # $HOME/tachikoma-data-for-user
    require 'bundler/setup'
    require 'tachikoma'
    require 'tachikoma/tasks'

    Tachikoma.root_path = File.expand_path(__FILE__) # resolved to `$HOME/tachikoma-data-for-user`
    # Tachikoma.data_path #=> `$HOME/tachikoma-data-for-user/data`
Replace hard coded `repos` to `Tachikoma.repos_path`
Merge pull request #7 from kyanny/feature/kyanny-refactor_path_handling

Bugfix: fix wrong `@data_path` resolution with path handling refactoring