Convert the build and install chapter to AsciiDoc.

Add OS X content. Remove GTK+ content. Update the names in the NSIS
package to match the documentation (untested).

Change-Id: Id8fd08982bc26871bb8a319b0319808bcdba878c
Reviewed-on: https://code.wireshark.org/review/3366
Reviewed-by: Gerald Combs <gerald@wireshark.org>