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.