Feature/task attribute

addresses #12, #9

This pull request contains the following changes.

#### 1. Support multiple prefix rule
```perl
rule '.o' => ['.c', '.h'] => sub { ... };
```

#### 2. Clarify how to access task attributes
All tasks have `name` and `sources` attributes, and can access the first `sources` element by `source` method.
```perl
task hoge => 'hoge.txt' => sub {
    my ($task, @argv) = @_;
    $task->name;    # hoge
    $task->sources; # ['hoge.txt']
    $task->source;  # 'hoge.txt'
};
file 'hoge.txt' => sub {
    my $task = shift;
    $task->name;    # README
    $task->sources; # []
    $task->source;  # undef
};
rule '.o' => ['.c', '.h'] => sub {
    my $task = shift;
    # say, invoke this rule task by: daiku hello.o
    $task->name;    # hello.o
    $task->sources; # ['hello.c', 'hello.h']
    $task->source;  # 'hello.c'
    sh "gcc", "-o", $task->name, "-c", $task->source;
};
```

Actually I'm not sure the attribute names are appropriate or my implementation is good.
@Songmu How do you think?