Add runtime strategy 'IgnoresAccessChecksTo'

[krafs]

This adds the concept of runtime strategies to Publicizer - ways to make sure the runtime doesn't throw a MemberAccessException when accessing a member in an assembly that was publicized at compile time.

There are two strategies:

Unsafe
This is what we have had all along. Publicizer has been setting <AllowUnsafeBlocks>true</AllowUnsafeBlocks>, which causes some runtimes (mainly Mono) to suppress access checks at runtime.

IgnoresAccessChecksTo
This emits an IgnoresAccessChecksToAttribute to the user's assembly, which causes some runtimes (most non-Mono) to suppress access checks.

The runtime strategy can be chosen in the project file by specifying it in the PublicizerRuntimeStrategies property.
By default, both of these strategies are used:

<PropertyGroup>
    <PublicizerRuntimeStrategies>Unsafe;IgnoresAccessChecksTo</PublicizerRuntimeStrategies>
</PropertyGroup>

Users should generally not have to do anything for their assemblies to work in the runtime by default. Picking a specific runtime strategy should only be for people who needs more control of how their assemblies are compiled, e.g. omitting the IgnoresAccessChecksToAttribute.

[krafs]

Closes #12.
Closes #13.