Fix link to Wiki and some typos in Downloads section

This updates the Wiki link from trac.gajim.org to dev.gajim.org. Fixes #11 (closed)

This also fixes a typo in the Downloads section (Debian and Ubuntu) where it states in the install instructions:

sudo apt-get install gajim-nightlyn

with an "n" at the end instead of:

sudo apt-get install gajim-nightly

Additionally, this puts an uppercase letter at the beginning of each sentence of the install instructions.