rpm: Add a option to insert raw Require statements

This can be useful to have requirements for different stages.

Examples:

    --rpm-add-require 'Requires(post): /usr/sbin/alternatives'
    --rpm-add-require 'Requires(preun): /usr/sbin/alternatives'
Merge branch 'feature/rpm-custom-requires' into develop
Added tests. Rebuild of example-1.0.0.gem required though
Updated the example gem with as little changes as possible

checksums.yaml.gz has been stripped as in the current gem.

Changes:

```diff
diff --git a/spec/fixtures/gem/example/example-1.0.gem b/spec/fixtures/gem/example/example-1.0.gem
index 0241779..9238a4a 100644
--- a/spec/fixtures/gem/example/example-1.0.gem
+++ b/spec/fixtures/gem/example/example-1.0.gem
@@ -46,10 +46,12 @@ dependencies:
         version: '0'
 description: sample description
 email: sample email
-executables: []
+executables:
+- example
 extensions: []
 extra_rdoc_files: []
-files: []
+files:
+- bin/example
 homepage: http://sample-url/
 licenses: []
 post_install_message:
@@ -78,3 +80,5 @@ test_files: []
 has_rdoc:
 ============== checksums =============
 =============== files ================
+bin/example
+#!/usr/bin/env ruby

```

See http://vstone.eu/diffing-gems-in-git/
Merge branch 'bugfix/fix_version_bins_with_no_bins' into develop