Update london:body-class.js

[donwojtallo]

Add multiple classes for Iron Router. Replace dot with underscore.

[donwojtallo]

This is an alternative approach to this pull request: #7
I avoid using dot in class names (I knot I could escape dot with backslash i CSS)
and I prefer using '_' instead of '-', because my project have paths like /user-management/settings. Additionally this code generates classes for all parent directories.
Eg. for url='/page/subpage/another-subpage'
it returns 'page page_subpage page_subpage_another-subpage'