Add Integer class for edalize
Add Integer class for edalize

This commit add basic Integer class to support Integer type parameters
for backends defined in edalize. For example: jobs parameter of Vivado.
Please see Issue #515 https://github.com/olofk/fusesoc/issues/515

olofk/fusesoc,olofk/fusesoc,lowRISC/fusesoc,lowRISC/fusesoc