Heap size in MB

This example is from the README:

```ruby
require 'jvmargs'
java_opts = JVMArgs::Args.new("-Xmx512M", "-XX:MaxPermSize=256m"){jmx true} 
java_opts.heap_size("2048M")
java_opts.permgen("256M")
```
The #heap_size method doesn't accept sizes in MBs and #permgen method does not exist. Both fixed in this PR.